The invention relates generally to stability control of electrical windfarm systems within an electric power system and more specifically to control of dynamic and voltage stability of an electric power system by controlling electrical windfarm systems.
Typically, an electric power system includes a plurality of power generation assets, which are spread over a geographic area. The electric power system also includes systems that consume power (loads) that may also be spread over the geographic area. The electric power system also includes a grid, a network of electric power lines and associated equipment used to transmit and distribute electricity over a geographic area. The infrastructure of the grid, may include, but is not limited to devices for interconnection, control, maintenance, and improvement of the electric power system operation. Typically, the electric power system includes a centralized control system operatively connected to the power generation assets for controlling a power output of each of the power generation assets, for example, using processing logic. The network operator usually operates the centralized control system. The power output of the power generation assets controlled by the centralized control system may include, but is not limited to an amount of electrical power, and a voltage for the electrical power.
The power generation assets include individual power generating stations. The power generating station, may for example, each serve a geographic region within the grid by delivering electrical power to such regions. The power generation assets may each include any type of power source. For example, the power generation assets may include a power source that generates electrical power at least partially from coal, water, a combustible fluid such as gasoline, natural gas, diesel fuel, etc., nuclear, wind, and solar energy.
Wind energy is often used to generate electrical power at power plants, often referred to as windfarms, using, for example, the rotation of large wind turbines to drive electrical generators. Windfarms and their associated windfarm controllers can control reactive power supply, and to a more limited extent active power. Larsen, in U.S. Pat. No. 7,119,452, U.S. Pat. No. 7,166,928, and U.S. Pat. No. 7,224,081, describes a voltage control for wind generators including a farm-level controller with a reactive power command and a wind turbine generator control system. Wind turbine generator voltage control may be provided by regulating the voltage according to a reference set by a higher-than-generator-level (substation or farm level) controller. Reactive power may be regulated over a longer term (e.g. few seconds) while wind turbine generator terminal voltage is regulated over a shorter term (e.g. fraction of a second) to mitigate the effect of fast grid transients.
For economic reasons and as one of the approaches to reduce the environmental impacts of fossil fuel power generation, wind turbine generators with larger power output are being produced and windfarms with greater numbers of wind turbine generators are being brought into operation. The power output from the windfarms in the future may comprise a significantly larger part of the total power being supplied and transmitted along the transmission grid. At the same time, there is increasing concern about the transmission capacity available for new large-scale windfarms, and the stability issues limiting transmission capacity.
The maximum operating capacity of transmission systems can often be limited by voltage stability, voltage limits, and electromechanical oscillations rather than by thermal loading limits. If these constraints can be overcome, network assets can be better utilized and in some cases investment in additional assets can be avoided.
FIG. 1 illustrates a representative response curve for a transmission system collapse due to increased power transfer between areas and how application of inventive controls for wind farms may extend the operation of the transmission system, preventing voltage collapse. The vertical axis represents transmission system voltage power flow 175 and the horizontal axis represents power flow 180. In a stressed condition, as the power flow (P) 175 increases, the voltage 180 at a given point on the voltage vrs. power flow line (solid line) 185 starts to decrease. The power P that can be transmitted in the system must be limited below Pmax1 194 due to voltage collapse. If the power increases close to Pmax1 194, the voltage 180 “collapses” considerably, normally causing protective relaying operations and load disconnections. In practice, an operating margin (not shown) is maintained below Pmax1 194 to avoid collapse.
Electromechanical oscillation modes that can occur, can be local, where individual synchronous generators in the power generation assets oscillate against the electric power system (of the order 0.3 to 1 Hz), inter-area, where groups of generators oscillate over a long distance system oscillates (0.2 to 1.0 Hz), and complex oscillations which can be a combination of local and inter-area modes.
Electromechanical oscillations will most likely occur within traditional generators in the network. Present wind turbine generator (WTG) technologies do not participate on the oscillation (although they can affect their damping). Further, potential new WTG concepts (hydraulic torque converters with synchronous machines directly connected) could actively participate in the electromechanical oscillation modes.
Damping of electromechanical oscillations can be improved with control devices in excitation of synchronous generators or power system stabilizer (PSS) controls. PSS have been applied in conventional power generation.
Flexible AC transmission systems (FACTS) devices are solutions to many transmission system problems. These devices often consist of the addition of series or shunt capacitors in combination with other passive devices or active switches may be added to the transmission grid. Typical FACTs devices are static VAR compensator (SVC) and Static synchronous compensator (STATCOM). The SVC and STATCOM are electrical devices for providing fast-acting reactive power compensation on high voltage electrical transmission networks. Other solutions consist of building additional lines and utilizing series capacitors in long lines. However, the FACTS approaches incur the cost for the hardware fixes.
Accordingly, there is a need to provide a structure and method for preventing voltage collapse on a transmission system, where the voltage collapse results from stressed conditions on the transmission line. Further, there is a need for improving damping of electromechanical oscillations within the electric grid.